The present invention relates to ceramic dielectric compositions which have high dielectric constants (K), e.g., between about 4900 and about 5400; low dissipation factors (DF), e.g., below about 2%; high insulation resistance (R) capacitance (C) products (RC), e.g., above about 7000 ohm-farads at 25.degree. C. and above about 3000 ohm-farads at 125.degree. C.; and stable temperature coefficient (TC) characteristics in which the dielectric constant does not alter from its base value at 25.degree. C. by more than about plus or minus 15% over a temperature range from -55.degree. C. to 125.degree. C.
Multilayer ceramic capacitors (MLCS) are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder; placing thereupon conducting metal electrode layers, usually a paladium/silver alloy in the form of metallic paste; stacking the resulting elements to form the multilayer capacitor; and firing to densify the material thus forming a multilayer ceramic capacitor. Other processes for forming MLCS are described in U.S. Pat. Nos. 3,697,950 and 3,879,645.
A high dielectric constant is important, because it allows a manufacturer to build smaller capacitors for a given capacitance. The electrical properties of many dielectric ceramic compositions may vary substantially as the temperature increases or decreases, however, and the variation of the dielectric constant and the insulation resistance with temperature and the dissipation factor, are also important factors to be considered in preparing ceramic compositions for use in multilayer capacitors.
In a desirable dielectric ceramic composition for use in a multilayer capacitor for applications requiring stability in the dielectric constant over a wide temperature range, the dielectric constant does not change from its base value at 25.degree. C. (room temperature) by more than about plus or minus 15%. The insulation resistance and capacitance product of such a composition should be more than 1000 ohm-farads at 25.degree. C. and more than 100 ohm-farads at maximum working temperature, 125.degree. C. in most cases. In addition, the dissipation factor should be as close to 0% as possible.
The method commonly used to produce such temperature stable capacitors consist of firing BaTiO.sub.3, used because of its high dielectric constant, together with minor ceramic oxide additives (dopants) which comprise minor amounts of elements or compounds which control the final dielectric properties. The degree of distribution of the ceramic oxide dopants throughout the barium titanate in the unfired state will determine such things as the extent of solid solution development during firing, grain growth, and the composition of the final fired grain and grain boundary. Thus, the efficiency of mixing is a key factor in the process to achieve the desired electrical properties in the finished multilayer ceramic capacitor. Until the present invention, however, the very minor amounts of ceramic oxide dopants have been very difficult to distribute in a homogeneous fashion throughout the blended ceramic dielectric composition.
It is well known that, in order for compositional development to take place during the firing stage of the manufacture of a multilayer ceramic capacitor, the particles of the ceramic oxide dopants of a dielectric composition must be in finely divided form to ensure adequate mixing of the ceramic oxide dopants with the BaTiO.sub.3. Ideally, in order for complete compositional development to take place during sintering of the ceramic dielectric composition, it is understood that the minor components must disperse themselves such that the environment around each barium titanate grain is the same throughout the bulk of the composition and such that the environment within each barium titanate grain is the same throughout the bulk of the composition. Typically, this is attempted by milling the components of the composition to a particle size of approximately 1 micron. Homogeneous distribution will be enhanced, however, by introducing ceramic oxide dopants of a smaller particle size, e.g., approximately 0.1 micron while continuing to use BaTiO.sub.3 particles of 1.0 micron in size. By way of illustration, using uniformly distributed powders of approximately 1 micron in spherical shape, it can be calculated that a unit of mix, prepared according to the proportions disclosed in the present invention, would contain 400 particles of barium titanate, 5 particles of niobium pentoxide and 1 particle of cobalt oxide. If, however, barium titanate powder of approximately 1.0 micron average particle size is mixed with niobium pentoxide and cobalt oxide of approximately 0.1 micron particle size, and assuming that these particles are perfectly spherical and uniformly distributed, it can be calculated that a unit of mix would contain 400 particles of barium titanate, 5000 particles of niobium pentoxide and 1000 particles of cobalt oxide. Thus each barium titanate particle would be surrounded by approximately thirteen niobium pentoxide particles and 3 cobalt oxide particles. It would therefore be expected that compositional development during sintering would occur much more efficiently and the effectiveness of the ceramic oxide dopant additives would be greatly enhanced compared to that achieved by mixing 1 micron particles of the minor components.
It is well known in the art that ceramic oxide particles can be reduced in size to about 1 micron by milling techniques. Before the present invention, however, it has been impossible to produce finely divided powders of the order of 0.1 microns because these milling techniques incur the risk of increasing the contamination levels of undesirable species, present in the milling media, and because milling efficiencies are significantly reduced as the particle size of the powder reaches submicron levels. The process described in this invention provides a means of enhancing the uniformity of the distribution of the minor component dopants in the ceramic mixture before firing, and thus enhancing the compositional development during sintering, by precipitating the minor component dopants in a finely divided form of approximately 0.1 micron average particle size in a controlled manner.
The process described in this invention has the advantage of producing ceramic oxide particles of the order of 0.1 microns without the problems associated with current milling techniques.
A second advantage of the process is the production of ceramic dielectric compositions with improved electrical properties, i.e., higher dielectric constants, lower dissipation factors and higher insulation resistance capacitance products than those processed by conventional mixing techniques. The higher dielectric constant achieved as a result of this process has the important advantage of allowing capacitor manufacturing companies to produce multilayer ceramic capacitors with higher capacitance values for a given chip size, or the same capacitance values at a reduced chip size, given that the number of active insulating layers and the thickness of each insulating layer are constants. The benefits are thus reduced cost and/or miniaturization.